The present invention relates to a belt pretensioner for a vehicle seat belt system and, in particular, to a pretensioner for tightening the belt by rotating the belt reel shaft of a belt retractor in the belt-winding direction.
Some vehicle seat belt systems have a pretensioner for tightening a belt worn by an occupant of the vehicle in order to tightly restrain the body of the occupant in an emergency and to prevent the occupant from undergoing secondary collision within the vehicle. Such a pretensioner is disclosed in Japanese Provisional Patent Publication No. 1-168547.
In the pretensioner of Japanese Publication No. 1-168547, a plurality of segmental arcuate pieces arranged in a circle around a belt reel shaft of a retractor are normally held in clearance from the shaft so that the retractor can function normally. When a cable wound around the pieces is pulled by a power source, the pieces are forced into engagement with the reel shaft and transmit displacement of the cable to the reel shaft such as to wind a portion of the belt onto the belt reel of the retractor. In this unit, an end portion of the cable is received in a groove in the casing that receives the pieces, and a braking load is applied to the end portion by a spring at a turn in the groove in order to maintain engagement between the pieces and the reel shaft and to transmit displacement of the cable to the reel shaft through the pieces.
In the above-described previously known pretensioner, the end portion of the cable to which the braking load is applied must be of a length at least equal to that of the stroke of the power source. Thus, space for accommodating this portion must be provided in the casing, and the requirement for such space prevents reducing the size and weight of the unit. To avoid this problem, it has been proposed, alternatively, to eliminate the end portion and, instead, attach the end of the cable directly to the pieces, but this makes the device less reliable in operation because the engagement is more unstable than in the case when the braking load is applied as described above.